The present invention provides methods and devices for measuring the concentration of an analyte in solution and relates to the field of chemistry.
Most conventional pH sensors on the market today utilize an ion sensitive glass bulb sensitive to pH and an internal reference electrode. The reference electrode is usually a chloridized silver (Ag|AgCl) wire immersed in potassium chloride (KCl) gel or liquid and separated from the sample being analyzed via a porous frit, as shown in the schematic in FIG. 1. Both the use of an internal reference electrode and the necessity for the inclusion of a porous frit impair the operation of these conventional glass pH probes due to problems with drift caused by changes in the reference electrode potential and fouling or blocking of the frit. Thus, these conventional pH probes require constant recalibration, the electrodes must be stored in a KCl solution to keep the porous frit from drying out, and the fragile glass membrane renders these probes unsuitable for many applications where pH measurement is required under conditions of high temperature or pressure.
Effort has been made to improve the function of the reference electrode by, for instance, modification of the electrode-analyte interface (see U.S. Pat. No. 7,276,142) or replacement of the typically heterogeneous redox couple (e.g. calomel or silver/silver chloride) with a homogeneous redox couple (e.g. iodide/triiodide) (see U.S. Pat. No. 4,495,050). These changes are based on the extension of the potentiometric reference electrode concept wherein the conventional reference electrode (CRE) typically comprises two halves of a redox couple in contact with an electrolyte of fixed ionic composition and ionic strength. Because both halves of the redox couple are present and the composition of all the species involved is fixed, the system is maintained at equilibrium and the potential drop (i.e. the measured voltage) across the electrode-electrolyte interface of the conventional reference electrode is then thermodynamically fixed and constant. The function of the reference electrode is then to provide a fixed potential to which other measurements, such as pH, may be compared.
While these conventional reference electrodes provide a stable potential, they suffer from many disadvantages. One disadvantage is the need for an electrolyte of fixed and known ionic composition and ionic strength, because any change in ionic composition or strength will result in a shift in equilibrium of the redox couple, thereby compromising the stability of the constant potential of the electrode. To preclude a change in electrolyte composition, the redox system and electrolyte are typically isolated from the sample under study via a porous frit or small aperture. This isolation introduces an additional disadvantage to the conventional reference electrode, namely the propensity for the frit or aperture to clog, rendering the electrode useless. These disadvantages are exacerbated by the fact that the electrolyte is typically an aqueous solution of high salt concentration, resulting in the requirement that the electrode frit or aperture must be kept wet to avoid clogging due to salt precipitation.
A remarkable advance in pH sensor technology is the solid state internally calibrated pH sensor comprised of two redox-active pH sensitive agents (anthraquinone (AQ) and 9,10-phenanthrenequinone (PAQ)) and one pH insensitive redox agent (e.g., Ferrocene (Fc)); see PCT Patent Publication Nos. 2005/066618 and 2007/034131 and GB Patent Publication No. 2409902. In such sensors, all three redox agents may be mixed together with multiwalled carbon nanotubes (MWCNT), graphite powder and epoxy, and the resulting admixture cured and formed into solid sensors. When a voltage sweep is applied to the sensor and the resultant current measured (using square wave voltammetry, for example), one observes three peaks: one peak for each of the three redox agents.
In these solid state internally calibrated pH sensors, the pH insensitive peak (due to Fc) should ideally be constant and independent of pH or ionic species in solution and should not drift over time. The AQ and PAQ peaks should ideally vary their position on the voltage sweep in a predictable fashion depending on the pH of the solution being measured. Finally, the positions of the pH sensitive peaks, when compared to the position of the pH insensitive peak, allow the solution pH to be deduced by comparing those values to a calibration table. For this system to have the greatest accuracy and so have the greatest scope of application, the pH insensitive peak must be stable over time, and its peak position must be unaffected by varying solution compositions. Otherwise, the accuracy of the system is compromised. Unfortunately, most if not all pH insensitive redox agents appear to be affected unsuitably by different ions and exhibit significant drift or shifts in peak position. This problem is also present in other sensors that respond to analytes other than pH. Accordingly, there remains a need in the art for materials and methods for making internally calibrated pH and other analyte sensors based on redox agents.
The present invention meets this need.